FEC block reconstruction system, method and computer program product for mitigating burst noise in a communications system

ABSTRACT

A system, method and computer program product is provided for mitigating the effects of burst noise on packets transmitted in a communications system. A transmitting device applies an outer code, which may include, for example, a block code, an exclusive OR (XOR) code, or a repetition code, to one or more packets prior to adaptation of the packets for transmission over the physical (PHY) layer of the communications system, wherein the PHY layer adaptation may include FEC encoding of individual packets. The outer coded packets are then separately transmitted over a channel of the communications system. A receiving device receives the outer coded packets, performs PHY level demodulation and optional FEC decoding of the packets, and then applies outer code decoding to the out6r coded packets in order to restore packets that were erased during transmission due to burst noise or other impairments on the channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to the following provisionalapplications: U.S. application Ser. No. 60/299,768, entitled “Method,System and Computer Program Product for Mitigating Burst Noise,” filedJun. 22, 2001, and U.S. application Ser. No. 60/308,101, entitled “FECBlock Reconstruction Method, System and Computer Program Product forMitigating BurstNoise,” filed Jul. 30, 2001. Each of these provisionalapplications is incorporated by reference in its entirety as if setforth fully herein.

This application is related to co-pending non-provisional applicationentitled “System, Method and Computer Program Product for MitigatingBurst Noise in a Communications System,” application Ser. No.10/175,330, filed Jun. 20, 2002, filed concurrently herewith andincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of communicationssystems. More specifically, the present invention relates tocommunication systems in which electronic information is transmitted inpackets.

2. Background

Communications systems maybe subject to intermittent, random bursts ofnoise, often referred to as burst noise or impulse noise. This burstnoise can result in the introduction of errors into the electronicinformation transmitted over the communications system. For example, ina communications system in which voice, data and/or other electronicinformation is transmitted in packets, the presence of burst noise cancause packets to become corrupted. Packet errors must be detected inorder for a communications system to operate reliably. Where packeterrors are detected but cannot be corrected, bad packets must be droppedand re-transmitted, resulting in a net loss of transmission speed andnetwork bandwidth. If the dropped packets include voice information, theresult may be a loss in overall quality of service (QOS).

Some communications systems utilize Forward Error Correction (FEC)and/or interleaving in order to control errors in transmitted dataresulting from burst noise. FEC involves the addition of extrainformation to data before it is transmitted, which is then used by thereceiver to detect errors and correct the data when necessary. Dependingon the FEC algorithm used, the receiver can detect and correct fewerthan a predetermined number of errors per packet. Different types of FECmethods include block codes, cyclic codes, Reed-Solomon codes, andconvolutional codes. Interleaving involves rearranging data to betransmitted according to a mapping and then returning the data to itsoriginal order at the receiver. Interleaving serves to spread theeffects of burst noise over multiple packets, thereby decreasing theaverage number of errors per packet. This, in turn, enhances the abilityof FEC to detect and correct errors.

For example, in a conventional cable modem system operating inaccordance with DOCSIS (Data Over Cable System Interface Specification),the upstream physical (PHY) layer utilizes a combination of Reed-Solomoncoding and interleaving to control errors. These methods allow anupstream receiver to correct errors resulting from burst noise of acertain duration and frequency of occurrence. However, there may beextreme cases where the burst noise is so severe that more errors areintroduced into a packet than these combined methods can detect andcorrect. In these instances, packets will be lost. Therefore, what isneeded is a technique for detecting and correcting packet errors incases where burst noise is more severe than can be handled byconventional PHY technology.

BRIEF SUMMARY OF THE INVENTION

The present invention addresses the above problems by providing noveltechniques for preventing the loss of packets due to burst noise in acommunications system. In accordance with embodiments of the invention,a transmitting device applies a layer of coding, which may be termed an“outer code,” to one or more packets prior to transmitting the packetsover a communication channel. The outer coding of the packets enables areceiving device to restore packets corrupted by burst noise ininstances where the packet errors could not otherwise be corrected byconventional PHY layer Forward Error Correction (FEC) and interleavingtechniques.

In a communications system in which data is transmitted in bursts, asevere impairment on the communication channel can cause an entire burstof data to be lost. One way to combat the impairment is to increase therobustness of an individual burst; for example, the amount of parityinformation in the burst may be substantially increased. However, thisapproach will negatively impact the data transmission efficiency of thecommunications system. Moreover, if the impairment is severe enough, theburst may be lost regardless of the increased level of error correction.In contrast, the outer coding techniques of the present inventionincrease robustness through the transmission of additional bursts (e.g.,block-encoded parity packets, repeated packets, or re-transmittedpackets), such that burst data may be recovered even in the instancewhere an entire burst has been lost due to an impairment on thecommunication channel.

Typically, a first aspect of a burst communication system operates toensure successful transmission of individual bursts of data over thecommunication channel, while a second aspect assembles data bursts intoa stream of data for processing by a higher-level application.Embodiments of the present invention may be advantageously implementedin conjunction with this second aspect to perform outer encoding anddecoding. In a DOCSIS system, the first aspect corresponds to the PHYlayer, while the second aspect corresponds to the MAC layer.Accordingly, the outer coding and decoding of packets may occur at theMAC layer of the transmitting and receiving devices, which resides justabove the PHY layer. For example, the PHY layer of a receiving devicemay be unable to correct packets which are in error due to burst noise.In accordance with embodiments of the present invention, these packetsare further processed by an outer code at the MAC layer in order tocorrect the bad packets.

In an embodiment of the invention the outer code comprises a block code,such as an (8, 4) Hamming code or an XOR code. A transmitting deviceapplies the block code to one or more packets to generate a plurality ofinformation segments and parity segments, each of which is thentransmitted separately over a communication channel to a receivingdevice. A receiving device receives and collects the information andparity segments and decodes them in accordance with the same block codein order to restore any segments that were lost during transmission.

In an alternate embodiment of the invention, the outer code is arepetition code in which the same packet is transmitted two or moretimes from a transmitting device to a receiving device. Provided atleast one of the repeated packets survives transmission, the receivingdevice can recover the original packet.

In another embodiment of the invention, a retransmission technique isused to mitigate the effects of burst noise on the communicationchannel. In accordance with this technique, a receiving device instructsa transmitting device to retransmit a packet which has been lost due tonoise on the communication channel. Methods for implementing thistechnique include sending data acknowledgment messages (ACKS) from thereceiving device to the transmitting device every time a packet isreceived successfully, or by sending data non-acknowledgment messages(NACKs) from the receiving device to the transmitting device when a badpacket is received.

In a further embodiment of the invention, an FEC block reconstructiontechnique is used by a receiving device to restore transmitted packetsthat have been corrupted by burst noise. This technique takes advantageof the fact that packets are often transmitted as a plurality of FECblocks. In accordance with such a technique, a receiving device receivessome but not all of the FEC blocks of a transmitted packet. Thereceiving device then replaces the bad FEC blocks with good FEC blocksfrom a repeated packet transmission, if repetition outer coding is used,or by requesting retransmission of the bad FEC blocks or the entireoriginal packet from a transmitting device, if a retransmissiontechnique is used. A combination of repetition coding, retransmission,and FEC block reconstruction may also be used.

Embodiments of the present invention provide a means for greatlyimproving the robustness of a communications system afflicted withoccasional, or bursty, extreme impairment levels. A particular advantageof the techniques provided herein is that they may be overlaid, in somecases, onto existing communication systems in the field with only minorupgrade or modification required. For example, embodiments of thepresent invention may be implemented primarily as a software applicationrunning in, or in conjunction with, existing communication devices, suchas existing cable modem or cable modem termination system (CMTS)hardware. Thus, embodiments of the present invention provide a solutionto a PHY level impairment without actually altering the PHY layer ofexisting systems.

Embodiments of the present invention may also provide for additionalrobustness above the PHY layer without having to rely on failuremechanisms from higher layers in the network protocol stack, and with aminimum impact on latency. Implementations of the present invention mayhave no noticeable impact on latency, unlike some Automatic RepeatRequest (ARQ) schemes. Embodiments of the present invention can providerobustness in cases where ARQ is not feasible, such as voiceapplications. By providing another level of mitigation against PHY layerimpairments, especially a severe time-varying impairment, embodiments ofthe present invention can optimize the conventional PHY layer for a morenormal, non-extreme impairment level. Embodiments of the presentinvention thus provide superior utilization of a communication channelas compared to a modified, traditional PHY-layer-only solution, whichmust somehow overcome severe impairments by itself. Finally, techniquesin accordance with the present invention can be combined with othertechniques, such as packet header suppression (PHS) techniques, tobeneficially result in increased robustness and efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 is a high level block diagram of a cable modem system in whichembodiments of the present invention may operate.

FIG. 2 is a schematic block diagram of an example cable modemtermination system (CMTS) in accordance with embodiments of the presentinvention.

FIG. 3 is a schematic block diagram of an example cable modem inaccordance with embodiments of the present invention.

FIG. 4 depicts a flowchart of a method for mitigating burst noise usingouter block coding in a transmitting device in accordance withembodiments of the present invention.

FIG. 5 depicts a flowchart of a method for mitigating burst noise usingouter block coding in a receiving device in accordance with embodimentsof the present invention.

FIG. 6 illustrates the use of a Hamming code technique for performingouter block encoding in accordance with embodiments of the presentinvention.

FIG. 7 illustrates the addition of identification bits to a plurality ofrelated information and parity segments in accordance with embodimentsof the present invention.

FIG. 8 depicts a flowchart of a method for mitigating burst noise usingrepetition coding in a transmitting device in accordance withembodiments of present the invention.

FIG. 9 depicts a flowchart of a method for mitigating burst noise usingrepetition coding in a receiving device in accordance with an embodimentof the present invention.

FIG. 10 depicts a flowchart of an alternate method for mitigating noiseusing repetition coding in a transmitting device in accordance withembodiments of the present invention.

FIG. 11 depicts a flowchart of a method for mitigating burst noise usingretransmission techniques in a transmitting device in accordance withembodiments of the invention.

FIG. 12 depicts a flowchart of a method for mitigating burst noise usingretransmission techniques in a receiving device in accordance withembodiments of the invention.

FIG. 13 depicts a flowchart of an alternate method for mitigating burstnoise using retransmission techniques in a transmitting device inaccordance with embodiments of the invention.

FIG. 14 depicts a flowchart of an alternate method for mitigating burstnoise using retransmission techniques in a receiving device inaccordance with embodiments of the invention.

FIG. 15 illustrates the use of a Hamming code technique for performingouter block encoding on FEC blocks in accordance with embodiments of thepresent invention.

FIG. 16 depicts a flowchart of a method for mitigating burst noise usingrepetition and FEC block reconstruction techniques in a transmittingdevice in accordance with embodiments of the invention.

FIG. 17 depicts a flowchart of a method for mitigating burst noise usingrepetition and FEC block reconstruction techniques in a receiving devicein accordance with embodiments of the invention.

FIG. 18 is a block diagram of an exemplary computer system forimplementing all or part of a method for mitigating burst noise inaccordance with embodiments of the present invention.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawings in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION OF THE INVENTION

Table of Contents

A. Example Cable Modem System

1. Example Cable Modem Termination System (CMTS)

2. Example Cable Modem

B. Methods for Mitigating Burst Noise in Accordance with Embodiments ofthe Present Invention

1. Outer Block Coding

-   -   a. Hamming Block Coding    -   b. Exclusive OR (XOR) Coding    -   c. Alternative Outer Block Coding Embodiments

2. Repetition Coding

3. Retransmission

4. Block Reconstruction

-   -   a. FEC Block Reconstruction and Outer Block Coding    -   b. FEC Block Reconstruction and Repetition Coding    -   c. Alternative FEC Block Reconstruction Embodiments

5. Outer Coding and Packet Header Suppression

6. Outer Coding of Downstream Transmissions in a Cable Modem System

7. Transmission Scheduling

C. Example Software Implementation

D. Conclusion

A. Example Cable Modem System

Although the techniques of the present invention may be practiced in anycommunications system, example embodiments described herein pertainspecifically to cable modem systems. Accordingly, an example cable modemsystem will be described in more detail below. However, the invention isnot limited to cable modem systems. Persons skilled in the art based onthe teachings provided herein will appreciate that the present inventionmay be practiced in any communications system in which information istransmitted in packets. For example, embodiments of the presentinvention may also be implemented in wireless communication systems thattransmit information in packets.

FIG. 1 is a high-level block diagram of an example cable modem system100 in which embodiments of the present invention may operate. Theexample cable modem system 100 enables voice communications, video anddata services based on a bidirectional transfer of packet-based traffic,such as Internet Protocol (IP) traffic, between a cable system head end102 and a plurality of cable modems 108 a–108 n over a hybridfiber-coaxial (HFC) cable network 106.

The cable headend 102 is comprised of at least one cable modemtermination system (CMTS) 104. The CMTS 104 is the portion of the cableheadend 102 that manages the upstream and downstream transfer of databetween the cable headend 102 and the cable modems 108 a–108 n, whichare located at the customer premises. The CMTS 104 broadcastsinformation downstream to the cable modems 108 a–108 n as a continuoustransmitted signal in accordance with a time division multiplexing (TDM)technique. Additionally, the CMTS 104 receives data from the cablemodems 108 a–108 n over a plurality of shared upstream channels. Datafrom the cable modems 108 a–108 n may be transmitted upstream inaccordance with a time domain multiple access (TDMA) technique or asynchronous code division multiple access (S-CDMA) technique. Inaccordance with TDMA, each cable modem 108 a–108 n may only sendinformation upstream as short burst signals during a transmissionopportunity allocated to it by the CMTS 104.

As shown in FIG. 1, the CMTS 104 further serves as an interface betweenthe HFC network 106 and a packet-switched network 112, transferring IPpackets received from the cable modems 108 a–108 n to thepacket-switched network 112 and transferring IP packets received fromthe packet-switched network 112 to the cable modems 108 a–108 n whenappropriate. The packet-switched network 112 may include, for example,the Internet.

In addition to the CMTS 104, the cable headend 102 may also include oneor more Internet routers to facilitate the connection between the CMTS104 and the packet-switched network 112, as well as one or more serversfor performing necessary network management tasks.

The HFC network 106 provides a point-to-multipoint topology for thehigh-speed, reliable, and secure transport of data between the cableheadend 102 and the cable modems 108 a–108 n at the customer premises.As will be appreciated by persons skilled in the relevant art(s), theHFC network 106 may include coaxial cable, fiberoptic cable, or acombination of coaxial cable and fiberoptic cable linked via one or morefiber nodes.

Each of the cable modems 108 a–108 n operates as an interface betweenthe HFC network 106 and at least one attached user device. Inparticular, the cable modems 108 a–108 n convert downstream signalsreceived over the HFC network 106 into IP data packets for receipt by anattached user device. Additionally, the cable modems 108 a–108 n convertIP data packets received from the attached user device into upstreamburst signals suitable for transfer over the HFC network 106.

In the example cable modem system 100, each cable modem 108 a–108 n isshown supporting only a single user device for clarity. In general, eachcable modem 108 a–108 n is capable of supporting a plurality of userdevices for communication over the cable modem system 100. User devicesmay include personal computers, data terminal equipment, telephonydevices, broadband media players, network-controlled appliances, or anyother device capable of transmitting or receiving data over apacket-switched network.

In the example cable modem system 100, cable modems 108 a–108 n and CMTS104 represent DOCSIS-compliant cable modem equipment. In other words,cable modems 108 a–108 n and CMTS 104 communicate in accordance withprotocols and formats set forth in the DOCSIS specification. The termDOCSIS (Data Over Cable System Interface Specification) generally refersto a group of specifications published by CableLabs that define industrystandards for cable headend and cable modem equipment. In part, DOCSISsets forth requirements and objectives for various aspects of cablemodem systems including operations support systems, management, datainterfaces, as well as network layer, data link layer, and physicallayer transport for data over cable systems.

1. Example Cable Modem Termination System (CMTS)

FIG. 2 depicts a schematic block diagram of an implementation of theCMTS 104 of cable modem system 100, which is presented by way ofexample, and is not intended to limit the present invention. The CMTS104 is configured to receive and transmit signals to and from the HFCnetwork 106, a portion of which is represented by the optical fiber 202of FIG. 2. Accordingly, the CMTS 104 will be described in terms of areceiver portion and a transmitter portion.

The receiver portion includes an optical-to-coax stage 204, an RF input206, a splitter 214, and a plurality of burst receivers 216. Receptionbegins with the receipt of upstream burst signals originating from oneor more cable modems by the optical-to-coax stage 204 via the opticalfiber 202. The optical-to-coax stage 204 routes the received burstsignals to the radio frequency (RF) input 206 via coaxial cable 205.These upstream burst signals have spectral characteristics within thefrequency range of roughly 5–42 MHz.

The received signals are provided by the RF input 206 to the splitter214 of the CMTS 104, which separates the RF input signals into Nseparate channels. Each of the N separate channels is then provided to aseparate burst receiver 216 which operates to demodulate the receivedsignals on each channel in accordance with a particular demodulationtechnique to recover the underlying information signals. Thedemodulation technique may comprise, without limitation, a QuadraturePhase Shift Key (QPSK) technique, an 8-Quadrature Amplitude Modulation(QAM) technique, a 16-QAM technique, a 32-QAM technique, or a 64-QAMtechnique. Each burst receiver 216 also converts the underlyinginformation signals from an analog form to digital form. This digitaldata is subsequently provided to the headend media access control (MAC)218.

The headend MAC 218 operates to process the digital data in accordancewith the DOCSIS specification. The functions of the headend MAC 218 maybe implemented in hardware or in software. In the example implementationof FIG. 2, the functions of the headend MAC 218 are implemented both inhardware and software. Software functions of the headend MAC 218 may bestored in either the random access memory (RAM) 220 or the read-onlymemory (ROM) 218 and executed by the CPU 222. The headend MAC is inelectrical communication with these elements via a backplane interface220 and a shared communications medium 232. In embodiments, the sharedcommunications medium 232 may comprise a computer bus or a multipleaccess data network.

The headend MAC 218 is also in electrical communication with theEthernet interface 224 via both the backplane interface 220 and theshared communications medium 232. When appropriate, Ethernet packetsrecovered by the headend MAC 218 are transferred to the Ethernetinterface 224 for delivery to the packet-switched network 112 via arouter.

The transmitter portion of the CMTS 104 includes a downstream modulator226, a surface acoustic wave (SAW) filter 228, an amplifier 230, anintermediate frequency (IF) output 212, a radio frequency (RF)upconverter 210 and the optical-to-coax stage 204. Transmission beginswith the generation of a digital broadcast signal by the headend MAC218. The digital broadcast signal may include data originally receivedfrom the packet-switched network 112 via the Ethernet interface 224. Theheadend MAC 218 outputs the digital broadcast signal to the downstreammodulator 226 which converts it into an analog form and modulates itonto a carrier signal in accordance with, for example, a 64-QAM or256-QAM technique.

The modulated carrier signal output by the downstream modulator 226 isinput to the SAW filter 228 which passes only spectral components of thesignal that are within a desired bandwidth. The filtered signal is thenoutput to an amplifier 230 which amplifies it and outputs it to the IFoutput 212. The IF output 212 routes the signal to the RF upconverter210, which upconverts the signal. The upconverted signal has spectralcharacteristics in the frequency range of approximately 54–860 MHz. Theupconverted signal is then output to the optical-to-coax stage 204 overthe coaxial cable 208. The optical-to-coax stage 204 broadcasts thesignal via the optical fiber 202 of the HFC network 106.

2. Example Cable Modem

FIG. 3 depicts a schematic block diagram of an implementation of thecable modem 108 a of cable modem system 100, which is presented by wayof example, and is not intended to limit the present invention. Thecable modem 108 a is configured to receive and transmit signals to andfrom the HFC network 106 via the coaxial connector 332 of FIG. 3.Accordingly, the cable modem 108 a will be described in terms of areceiver portion and a transmitter portion.

The receiver portion includes a diplex filter 302, an RF tuner 304, aSAW filter 306, an amplifier 308, and a downstream receiver 310.Reception begins with the receipt of a downstream signal originatingfrom the CMTS 104 by the diplex filter 302. The diplex filter 302operates to isolate the downstream signal and route it to the RF tuner304. In embodiments, the downstream signal has spectral characteristicsin the frequency range of roughly 54–860 MHz. The RF tuner 304downconverts the signal and outputs it to the SAW filter 306, whichpasses only spectral components of the downconverted signal that arewithin a desired bandwidth. The filtered signal is output to theamplifier 308 which amplifies it and passes it to the downstreamreceiver 310. Automatic gain controls are provided from the downstreamreceiver 310 to the RF tuner 304.

The downstream receiver 310 demodulates the amplified signal inaccordance with, for example, either a 64-QAM or 256-QAM technique torecover the underlying information signal. The downstream receiver 310also converts the underlying information signal from an analog form todigital form. This digital data is subsequently provided to the mediaaccess control (MAC) 314.

The MAC 314 processes the digital data, which may include, for example,Ethernet packets for transfer to an attached user device. The functionsof the MAC 314 may be implemented in hardware or in software. In theexample implementation of FIG. 3, the functions of the MAC 314 areimplemented in both hardware and software. Software functions of the MAC314 may be stored in either the RAM 322 or the ROM 324 and executed bythe CPU 320. The MAC 314 is in electrical communication with theseelements via a shared communications medium 316. The sharedcommunications medium may comprise, for example, a computer bus or amultiple access data network.

The MAC 314 is also in electrical communication with the Ethernetinterface 318 via the shared communications medium 316. Whenappropriate, Ethernet packets recovered by the MAC 314 are transferredto the Ethernet interface 318 for transfer to an attached user device.

The transmitter portion of the cable modem 108 includes an upstreamburst modulator 326, a low pass filter 328, a power amplifier 330, andthe diplex filter 302. Transmission begins with the reception of a datapacket by the MAC 314. The data packet may include data originallyreceived from an attached user device via the Ethernet interface 318.Alternatively, the data packet may be generated by the MAC 314 as partof the cable modem network management and upkeep. The MAC 314 formatsthe data packet in compliance with the protocols set forth in the DOCSISspecification. The MAC 314 outputs the data packet to the upstream burstmodulator 326 which converts it into analog form and modulates it onto acarrier signal in accordance with a particular modulation technique. Themodulation technique may comprise, without limitation, a QuadraturePhase Shift Key (QPSK) technique, an 8-Quadrature Amplitude Modulation(QAM) technique, a 16-QAM technique, a 32-QAM technique, or a 64-QAMtechnique.

The upstream burst modulator 326 outputs the modulated carrier signal tothe low pass filter 328 which passes signals with spectralcharacteristics in a desired bandwidth within the frequency range ofapproximately 5–42 MHz. The filtered signals are then introduced to thepower amplifier 330 which amplifies the signal and provides it to thediplex filter 302. The gain in the power amplifier 330 is regulated bythe burst modulator 326. The diplex filter 302 isolates the amplifiedsignal and transmits it upstream over the HFC network 106 during ascheduled burst opportunity.

B. Methods for Mitigating Burst Noise in Accordance with Embodiments ofthe Present Invention

1. Outer Block Coding

FIGS. 4 and 5 depict flowcharts 400 and 500 of methods for mitigatingburst noise using outer block coding in a transmitting device and areceiving device in accordance with embodiments of the invention. Theseflowcharts will be described in reference to an embodiment in which thetransmitting device is the cable modem 108 a described above inreference to FIG. 3, and the receiving device is the CMTS 104 describedabove in reference to FIG. 2. However, the invention is not so limited.For example, the transmitting device may be a CMTS and the receivingdevice may be a cable modem. Alternatively, other transmitting andreceiving devices may be used.

The method of flowchart 400 begins at step 402, in which the cable modem108 a receives one or more packets of information for transmission overan upstream channel to the CMTS 104. The packets may include voice, dataand/or other information. For example, the packets may comprise part ofa continuous stream of packets carrying voice information. In anembodiment, the packets arrive at the cable modem 108 a via the Ethernetinterface 318. In an alternate embodiment, the packets are generatedwithin the cable modem 108 a as part of the cable modem networkmanagement and upkeep.

At step 404, the one or more packets received at step 402 are collectedand reorganized into groups of k information segments. For shortpackets, this step may comprise simply collecting k packets andidentifying each packet as one of k information segments. For longerpackets, this step may comprise fragmenting a single packet into kparts, and identifying each part as one of k information segments.

At step 406, an outer block code is applied to the k informationsegments to generate parity segments. In an embodiment, step 406 entailsarranging the k segments as a series of columns and then performingrow-wise parity checks on the rows of the matrix formed by thearrangement of the k columns. As will be appreciated by persons skilledin the art, the arrangement of the k information segments into rows andcolumns is actually a conceptual step and may not comprise a literalprocess.

The outer block code applied in step 406 may be any of the many blockcodes available in the literature, such as an (n, k) binary code, and inparticular may be an (n, k) Hamming code. In an embodiment in which theouter code is an (n, k) binary code, each row includes a single bit fromeach of the k segments and parity check equations are used to computen–k additional parity bits for each row. These n–k parity bits arearranged to form an additional n–k columns, or parity segments. Thus,the parity checks for each row provide a row of bits for the newlycreated n–k parity segments. If there are m bits in each of the originalk segments, then there will be m parity check rows processed, and thenewly created n–k parity segments will each have m bits.

Step 408 is an optional processing step in which identification bits areadded to each of the k information segments and n–k parity segments. Theidentification bits may be used to uniquely identify informationsegments and parity check segments as originating from the same set of nencoded segments. The identification bits may also be used as a counterthat establishes the order in which the information segments and paritycheck segments were originally encoded. In an embodiment, theidentification bits may be repeated or otherwise placed in each columnwith redundancy to increase their robustness beyond the normal data bitsin each segment.

In an embodiment, each of the above steps 404 through 408 is performedby the MAC 314 of the cable modem 108 a.

At step 410, each of the k information segments and n–k parity checksegments is adapted for transmission over the physical (PHY) layer of acable modem system. In a DOCSIS cable modem system, this step includesprepending a MAC header to each segment, optionally performing FECencoding on the MAC header and the payload, and then prepending apreamble for subsequent burst transmission. As a result, each segmentbecomes the payload of a separate upstream burst. This step may alsoinclude interleaving of the upstream burst. In an embodiment, theprepending of a MAC header to each segment is performed by the MAC 314of the cable modem 108 a, while FEC encoding, prepending of a preamble,and interleaving is performed by PHY layer logic (not shown in FIG. 3)that resides between the MAC 314 and the upstream burst modulator 326 inthe cable modem 108 a.

At step 412, the cable modem 108 a separately transmits each burstgenerated in step 410 over an upstream communication channel of thecable modem system to the CMTS 104. In accordance with a DOCSIS cablemodem system, the cable modem 108 a must request a transmissionopportunity from the CMTS for each burst it wishes to transmit over theupstream channel. Burst transmission includes the conversion of eachburst into analog form and modulation onto a carrier signal by theupstream burst modulator 326 of the cable modem 108 a for transmissionduring an appropriate time slot.

The method of flowchart 500 begins at step 502, in which the CMTS 104receives the upstream bursts transmitted from the cable modem 108 aduring step 412 of flowchart 400. At step 504, a segment is recoveredfrom each upstream burst. This step includes demodulation of each burstand conversion to digital form by the burst receiver 216 of CMTS 104.This step may also include deinterleaving and FEC decoding of the burstdata by PHY layer logic within the CMTS that resides between the burstreceiver 216 and the headend MAC 218 (not shown in FIG. 2).

As shown at step 506, bad segments are identified, or marked, as aresult of the processing of step 504. The marking of a bad segment maybe referred to as erasure. A bad segment may be marked based on thereceipt of a bad burst preamble. Alternately, a bad segment may bemarked as a result of FEC decoding of the burst payload. In the lattercase, a bad segment will be marked if more errors are detected in asegment than can be corrected by the FEC technique being used. In aDOCSIS cable modem system, Reed-Solomon (RS) coding is used for FEC;however a parity check code may also be used. In either case, FEC has ahigh probability of detecting an error, even when it cannot correct theerror.

At step 508, the recovered segments are passed to the headend MAC 218 ofthe CMTS 104 where related information segments and parity segments arecollected for application of outer block code decoding. In order toproperly decode the segments, the n information segments and paritysegments that were originally encoded together must be regrouped in theorder in which they were encoded. However, these segments may notnecessarily be received by the CMTS 104 in the back-to-back sequence inwhich they were encoded, due to interspersed transmissions from othercable modems or even from other service flows generated by the samecable modem 108 a.

In an embodiment, the MAC 218 utilizes identification bits included ineach segment in order to collect related information and paritysegments. As discussed above in regard to optional step 408 of flowchart400, identification bits may be added to each segment to uniquelyidentify segments that originate from the same set of n encodedsegments. As also discussed above, the identification bits may alsoprovide a counter that establishes the order in which the informationsegments and parity segments were originally encoded. Alternatively, theMAC 218 may group segments by tracking the grant of upstream bursts tothe cable modem 108 a, which are relayed in a standard DOCSIS MAPmessage. As will be appreciated by persons skilled in the art, acombination of these techniques, and alternative techniques, may also beused to group the segments.

At step 510, the headend MAC 218 decodes the n related information andparity segments collected in step 508 in accordance with the outer blockcode originally used to encode them in step 406 of flowchart 400,discussed above in reference to FIG. 4. The outer block decoding step510 includes arranging the n information and parity segments as ncolumns, wherein any segments marked as erased in step 506 are placed aserased columns for decoding purposes. As will be appreciated by personsskilled in the art, the arrangement of the segments into columns isactually a conceptual step and may not comprise a literal process. Eachrow of the matrix formed by the columnar arrangement of the n segmentsis then decoded to recover bits from erased segments which appear ineach row. Decoding of the columns in this manner will also permit errordetection and correction of a limited number of individual bit errors insegments that have not been erased, wherein the number of bit errorsthat may be corrected is determined by the outer block code being usedand the number of erased segments. As a result of the decoding step 510,the original k information segments are recovered.

At step 512, the headend MAC 218 reorganizes the k information segmentsas necessary into one or more packets, reversing the process performedat step 402 of flowchart 400. For example, if each of the k informationsegments represent fragmented portions of a long packet, then the ksegments will be concatenated together to reconstitute the long packet.Alternately, in the case where each of the k information segmentsrepresents a single packet, no reorganization is performed. Therecovered packets are then available for further processing by theheadend MAC 218, which may, for example, forward the packets to apacket-switched network via the Ethernet interface 224.

a. Hamming Block Coding

An example of the methods of flowchart 400 and flowchart 500 in whichthe outer block code is an extended (8, 4) Hamming code will now bedescribed. For the purposes of this example, it is assumed that a queueof four m length data packets is collected at the cable modem forupstream transmission and organized as four information segments, sothrough s₄, respectively, in accordance with steps 402 and 404 offlowchart 400. As depicted in FIG. 6, the four information segments arearranged as a series of columns 602. In accordance with step 406 offlowchart 400, parity equations for an (8, 4) Hamming code are thenapplied to each row of four information bits created by the arrangementof columns 602 to calculate four parity bits for each row. For example,with continued reference to FIG. 6, parity check equations are appliedto the row of four information bits s₁d₁, s₂d₁, s₃d₁, and s₄d₁, togenerate the four parity bits s₅d₁, s₆d₁, s₇d₁ and s₈d₁. Each resultingrow of information and parity bits 606 a through 606 m represents aseparate (8, 4) Hamming codeword.

The generation of four parity bits for each row results in the creationof four new columns 604. These four new columns 604 correspond to fournew parity segments, s₅ through s₈

In accordance with step 408 of flowchart 400, identification bits may beadded to each segment s₁–s₈ to uniquely identify the segments asoriginating from the same set of encoded segments and/or to establishthe order in which the segments were originally encoded. FIG. 7illustrates the prepending of sequential identification bits 706 to theinformation segments s₁ through s₄ and the parity segments s₅ throughs₈, which results in extended information segments 702 and paritysegments 704. These identification bits identify the encoding order ofeach of the segments. Although not shown in FIG. 7, a uniqueidentification number identifying the set of packets may also be addedto each segment. Note that the rows created by the addition of theidentification bits 706 do not represent (8, 4) Hamming codewords.

Each of the segments s₁–s₈ is then adapted for transmission over the PHYlayer of a cable modem system in accordance with step 410 of flowchart400. As discussed above, this includes prepending a MAC header to eachsegment, optionally performing FEC encoding on the MAC header and thepayload, and then prepending a preamble for subsequent bursttransmission. Where FEC is performed, it may be used to encode theidentification bits as well as the data included in each segment. Eachsegment is then transmitted upstream in one of eight separate bursts ofupstream transmission using the DOCSIS PHY, in accordance with step 412of the flowchart 400.

In accordance with steps 502 and 504 of flowchart 500, the eightupstream bursts are received by the CMTS, which recovers the eightsegments s -s through the demodulation and optional de-interleaving andFEC decoding of the bursts. Due to an impairment in the upstreamchannel, one or more of these segments are marked as erased pursuant tostep 506 of flowchart 500.

In accordance with step 508 of the flowchart 500, the CMTS then uses theprepended identification bits 706 in each segment to re-assemble thesegments s₁–s₈ in the column arrangement depicted in FIG. 7. Any erasedsegments in the matrix are identified at this stage. The CMTS thendecodes each row of the assembled matrix using the same (8,4) Hammingcode scheme implemented in the cable modem to recover lost informationsegments. Because an (8, 4) Hamming code has been used, as long as (a)three or fewer segments are erased, or (b) at most one segment is erasedand at most one bit error occurs in any row of the remaining sevencolumns, then the original information segments can be recovered. In 54out of 70 cases, four of the eight segments can be erased and theoriginal four information segments can still be restored.

b. Exclusive OR (XOR) Coding

In an embodiment of the present invention, the encoding and decodingsteps of flowchart 400 and flowchart 500 are performed in accordancewith an exclusive OR (XOR) code. The XOR code is essentially a (6, 4)code scheme that is applied to four information segments. For example,the XOR code may be applied to four information segments that resultfrom the division of a single packet. Application of the XOR code tofour information segments results in the generation of two paritysegments. The first parity segment is the result of performing anexclusive OR operation on the data in the first and fourth informationsegments and the second parity segment is the result of performing anexclusive OR operation on the data in the second and third informationsegments. In accordance with this technique, any single informationsegment can be recovered and in 9 out of 15 cases, two missinginformation segments can be recovered.

c. Alternate Outer Block Coding Embodiments

In an alternate embodiment of the present invention, the informationsegments organized in step 404 of flowchart 400 need not be of the samelength. Thus, a plurality of packets of different lengths may bedirectly processed as information segments. For example, with referenceto the (8, 4) Hamming code example set forth above, assume that the fourinformation segments, s₁ through s₄, are each of a different length. Inaccordance with this alternate embodiment, the three shorter informationsegments are extended with fill zero data bits prior to row Hammingencoding. As a result, the four parity columns which are created willhave the length of the longest of the four original informationsegments. The fill zeroes may then be stripped off the four informationsegments after identification bits have been added so that the originalinformation segments are transmitted upstream without thenow-unnecessary extending fill bits.

At the CMTS, since five of the segments (the longest information segmentand each of the four parity segments) have the maximum length, and atleast four must make it successfully over the upstream channel to have achance of completely recovering the upstream data, the maximum datalength of the subset of eight segments successfully making it upstreamis used for constructing the matrix for Hamming decoding. Extending fillbits once again are added to the three shortest information segments (orthose shorter information segments which survive upstream transmission).In this fashion, the techniques of flowcharts 400 and 500 may beextended to different length information segments.

In a further embodiment, it may be desirable to transmit less than afull set of information segments from the cable modem. This may be thecase where the arrival time of packets at the cable modem is notguaranteed. For example, with further reference to the (8, 4) Hammingcode example set forth above, it may be desirable to proceed with theupstream transmission of segments even though fewer than a full set offour information segments are in the queue. In this case, completecolumns of zero filler data are added as part of step 404 of flowchart400, until four columns are created. Each of these filler columns has alength equal to the maximum length of the data packets which havearrived in the queue. In accordance with this embodiment, theidentification bits subsequently added to each segment, including thefour parity segments, in step 408, include a value describing the totalnumber of information segments in the set (one to four in thisembodiment). Then, these additional filler columns may be optionallydeleted or removed prior to adaptation for the PHY layer at step 410 tomaintain upstream efficiency.

At the CMTS, in decoding step 510 of flowchart 500, matrix assemblyincludes examining the total number of information segments described inthe identification bits of each successfully received segment. If themajority of the segments indicate that N information segments wereencoded, and N is less than 4, then 4−N filler columns of zero data areassembled beginning with column 4−N+1 and ending at column 4. Decodingthen proceeds as if a fall eight segments had been received.

2. Repetition Coding

FIGS. 8 and 9 depict flowcharts 800 and 900 of methods for mitigatingburst noise using repetition coding in a transmitting device and areceiving device in accordance with embodiments of the invention. Theseflowcharts will be described in reference to an embodiment in which thetransmitting device is the cable modem 108 a described above inreference to FIG. 3, and the receiving device is the CMTS 104 describedabove in reference to FIG. 2. However, the invention is not so limited.For example, in an embodiment, the transmitting device may be a CMTS andthe receiving device may be a cable modem. Alternatively, othertransmitting and receiving devices may be used.

The method of flowchart 800 begins at step 802, in which the cable modem108 a receives a packet of information for transmission over an upstreamchannel to the CMTS 104. The packet may include voice, data and/or otherinformation. For example, the packet may comprise part of a continuousstream of packets carrying voice information. In an embodiment, thepacket arrives at the cable modem 108 a via the Ethernet interface 318.In an alternate embodiment, the packet is generated within the cablemodem 108 a as part of the cable modem network management and upkeep.

At step 804, the packet received at step 802 is copied, or repeated, togenerate a total of N packets, where N is an integer of 1 or more.Although any value of N may be used in accordance with embodiments ofthe present invention, practical values for N, taking into accountnetwork bandwidth concerns, may include N=2 or N=3. In an embodiment,step 804 is performed by the MAC 314 of the cable modem 108 a.

At step 806, the original packet received in step 802 and the repeatedpackets generated in step 804 are each adapted for transmission over thephysical (PHY) layer of a cable modem system. In a DOCSIS cable modemsystem, this step includes prepending a MAC header to each packet,optionally performing FEC encoding on the MAC header and the payload,and then prepending a preamble for subsequent burst transmission. As aresult, each packet becomes the payload of a separate upstream burst.This step may also include interleaving of the upstream burst. In anembodiment, the prepending of a MAC header to each segment is performedby the MAC 314 of the cable modem 108 a, while FEC encoding, prependingof a preamble, and interleaving is performed by PHY layer logic (notshown in FIG. 3) that resides between the MAC 314 and the upstream burstmodulator 326 in the cable modem 108 a.

At step 808, the cable modem 108 a separately transmits each burstgenerated in step 806 over an upstream communication channel of a cablemodem system to the CMTS 104. Burst transmission includes the conversionof each burst into analog form and modulation onto a carrier signal bythe upstream burst modulator 326 of the cable modem 108 a fortransmission during an appropriate time slot.

In accordance with a DOCSIS cable modem system, to ensure that repeatedpackets have allocated minislots (i.e., grants) for upstreamtransmission, either the cable modem 108 a must request additionalminislots from the CMTS 104, or the CMTS must automatically make thenecessary additional grants at some point later in time for repeatedpackets. The time delay between grants for a given repeated packet maybe constant, or random, based on efficiently filling the availableDOCSIS MAP space. Alternatively, the time delay may be based on avoidingperiodic impulse noise. For example, in a case of repetition codingwhere N=3, the spacing between the first and second grant may bedifferent from the spacing between the second and third grant. Thisprevents the case of periodic impulses obliterating all three packetsdue to a synchronization between the periodic impulses and the repeatedpackets. The optimal spacing between repeated packets will depend on theassumed impulse interarrival time statistics. For CMTS systems whichsample, detect, and classify impulse events, this information may beused by the CMTS to determine the optimal spacing of repeated grants.Such a system is disclosed in commonly-owned patent application Ser. No.09/988,065, entitled “Method and Apparatus for the Detection andClassification of Impairments on an RF Modulated Network,” filed Nov.16, 2001, incorporated by reference in its entirety herein.

The method of flowchart 900 begins at step 902, in which the CMTS 104receives the upstream bursts transmitted from the cable modem 108 aduring step 808 of flowchart 800. At step 904, a packet is recoveredfrom each upstream burst. This step includes demodulation of each burstand conversion to digital form by the burst receiver 216 of CMTS 104.This step may also include deinterleaving and FEC decoding of the burstdata by PHY layer logic within the CMTS that resides between the burstreceiver 216 and the headend MAC 218 (not shown in FIG. 2).

As shown at step 906, bad packets are identified, or marked, as a resultof the processing of step 904. The marking of a bad packet may bereferred to as erasure. A bad packet may be marked based on the receiptof a bad burst preamble. Alternately, a bad packet may be marked as aresult of FEC decoding of the burst payload. In the latter case, a badpacket will be marked if more errors are detected in a packet than canbe corrected by the FEC technique being used. In a DOCSIS cable modemsystem, Reed-Solomon (RS) coding is used to encode the burst payload;however a parity check code may also be used. In either case, FEC has ahigh probability of detecting an error, even when it cannot correct theerror.

At step 908, the recovered packets are passed to the headend MAC 218 ofthe CMTS 104, which compares successfully received adjacent packets froma given cable modem service flow and eliminates duplicates, if any,based on the assumption that that they are simply repetitions. Inaccordance with this technique, provided at least one of the N packetstransmitted from the cable modem 108 a has not been erased, the headendMAC 218 will able to recover the original packet.

Note that in an alternate embodiment of the present invention, therepetition step 804 of flowchart 800 could be moved to follow the stepof adapting a packet for the PHY layer. This alternate embodiment isillustrated in flowchart 1000 of FIG. 10. As shown in FIG. 10, step 1002includes receiving a packet for transmission over a communicationchannel, step 1004 includes adapting the packet for the PHY layer,resulting in the generation of a burst, and step 1006 includes copying,or repeating the burst, to generate a total of N bursts, where N is aninteger of 1 or more. The original and repeated bursts are thenseparately transmitted over the communication channel at step 1008.

3. Retransmission

FIGS. 11 and 12 depict flowcharts 1100 and 1200 of a method formitigating burst noise using retransmission techniques in a transmittingdevice and a receiving device in accordance with embodiments of theinvention. These flowcharts will be described in reference to anembodiment in which the transmitting device is the cable modem 108 adescribed above in reference to FIG. 3, and the receiving device is theCMTS 104 described above in reference to FIG. 2. However, the inventionis not so limited. For example, in an embodiment, the transmittingdevice may be a CMTS and the receiving device may be a cable modem.Alternatively, other transmitting and receiving devices may be used.

In accordance with the method of flowchart 1100, the cable modem 108 atransmits a packet over an upstream channel to the CMTS 104 at step1102. At step 1104, the cable modem 108 a waits for a predeterminedamount of time to receive an acknowledge (ACK) message over a downstreamchannel from the CMTS 104 that indicates that the packet transmitted instep 1102 was received successfully (in other words, it was received andnot determined to be a bad packet by the CMTS 104). If an ACK message isreceived within the predetermined amount of time, then the transmissionof the packet is complete, as indicated at step 1106. However, if an ACKmessage is not received within the predetermined amount of time, thenthe cable modem 108 a retransmits the packet, which is indicated by thearrow returning to step 1102 from step 1104 in flowchart 1100. In anembodiment of the present invention, the MAC 314 of the cable modem 108b is responsible for determining whether or not to retransmit a packet.

In an embodiment of the present invention, the cable modem 108 acontinues to retransmit a packet until such time as an appropriate ACKmessage is received from the CMTS 104. In an alternate embodiment, thecable modem 108 a retransmits a packet only a fixed number of timesbefore abandoning transmission of the packet. In such an embodiment, oneor more retransmission may be attempted.

The method of flowchart 1200 begins at step 1202, in which the CMTS 104receives the packet transmitted by the cable modem 108 a in step 1102 offlowchart 1100. At step 1204, the CMTS 104 determines if the packet isbad. For example, in an embodiment, the CMTS 104 identifies the packetas bad when it includes errors that cannot be corrected by FEC or if ithas a bad burst preamble. If the packet is bad, then the CMTS 104discards the packet as shown at step 1206. However, if the packet is notbad then the CMTS 104 sends an ACK message to the cable modem 108 aindicating that the packet was received successfully, and continues toprocess the packet as shown at step 1208.

In an alternate embodiment of the present invention, a retransmissiontechnique is implemented that utilizes non-acknowledgment messages(NACKs) rather than ACKs for determining when to retransmit a packet.This alternate embodiment is illustrated in flowcharts 1300 and 1400 ofFIGS. 13 and 14, respectively.

In accordance with the method of flowchart 1300, the cable modem 108 atransmits a packet over an upstream channel to the CMTS 104 at step1302. At step 1304, the cable modem 108 a waits for a predeterminedamount of time to receive a NACK message from the CMTS 104 over adownstream channel that indicates that the packet transmitted in step1302 has not been received successfully. If a NACK message is notreceived within the predetermined amount of time, then the transmissionof the packet is complete, as indicated at step 1306. However, if a NACKmessage is received within the predetermined amount of time, then thecable modem 108 a retransmits the packet, which is indicated by thearrow returning to step 1302 from step 1304 in flowchart 1300. Thepacket may be continually retransmitted until such time as a NACKmessage is not received, or may be retransmitted only a fixed number oftimes.

The method of flowchart 1400 begins at step 1402, in which the CMTS 104receives the packet transmitted by the cable modem 108 a in step 1302 offlowchart 1300. At step 1404, the CMTS 104 determines if the packet isbad. For example, in an embodiment, the CMTS 104 identifies the packetas bad when it includes errors that cannot be corrected by FEC or if ithas a bad burst preamble. If the packet is bad, then the CMTS 104discards the packet and sends a NACK message to the cable modem 108 aindicating that the packet was not received successfully as shown atstep 1406. However, if the packet is not bad then the CMTS 104 continuesto process the packet as shown at step 1408.

4. FEC Block Reconstruction

Embodiments of the present invention may take advantage of the fact thatpackets are often transmitted as two or more FEC encoded blocks, or “FECblocks.” For example, in a DOCSIS cable modem system, a cable modem mayencode a packet using Reed Solomon (RS) techniques to produce two ormore FEC blocks, each of which form part of the same upstream burst.During transmission, burst noise on the upstream channel may interferewith some, but not all, of the FEC blocks. Consequently, the CMTS mayrecover some of the FEC blocks, while erasing others. As will bedescribed in more detail below, the ability of the CMTS to recover atleast a portion of a plurality of FEC blocks in a burst permits the CMTSto restore lost information in a manner that is more efficient than ifthe entire packet were erased.

a. FEC Block Reconstruction and Outer Block Coding

The fact that the CMTS may recover one or more of the FEC blocks of apacket that has been impacted by burst noise can enhance theeffectiveness of the outer block coding technique described above inreference to FIGS. 4 and 5. To illustrate this concept, assume that theouter block code is the (8, 4) Hamming block code described above, whichpermits four information segments to be recovered by the CMTS providedthat no more than three of the eight transmitted segments (fourinformation segments plus four parity segments) are erased.

However, assume that each of the information and parity segments in thisexample has been FEC encoded to generate a first FEC block followed by asecond FEC block. This is illustrated in FIG. 15, in which informationsegment s₁ has been FEC encoded to generate FEC blocks s₁f₁ and s₁f₂,information segment s₂ has been FEC encoded to generate FEC blocks s₂f₁and s₂f₂, information segment s₃ has been FEC encoded to generate FECblock s₃f₁ and s₃f₂, and information segment s₄ has been FEC encoded togenerate FEC blocks s₄f₁ and s₄f₂. Likewise, parity segment s₅ has beenFEC encoded to generate FEC blocks s₅f₁ and s₅f₂, parity segment s₆ hasbeen FEC encoded to generate FEC blocks s₆f₁ and s₆f₂, parity segment s₇has been FEC encoded to generate FEC blocks s₇f₁ and s₇f₂, and paritysegment s₈ has been FEC encoded to generate FEC block s₈f₁ and s₈f₂.

In accordance with this example, as long as no more than three of theeight first FEC blocks in each segment are bad, then the first FEC blockin each information segment may be recovered, and as long as no morethan three of the eight second FEC blocks in each segment are bad, thenthe second FEC block in each information segment may be recovered. Thus,with continued reference to FIG. 15, as long as no more than three ofthe first FEC blocks s₁f₁ through s₈f₁ are bad, then the first FECblocks s₁f₁ through s₄f₁ in each information segment s₁ through s₄ maybe recovered. Likewise, as long as no more than three of the eightsecond FEC blocks s₁f₂ through s₈f₂ are bad, then the second FEC blockss₁f₂ through s₄f₂ in each information segment s₁ through s₄ may berecovered. As a result, there may be instances where as many as six outof the eight segments transmitted have a bad FEC block due to burstnoise, but nevertheless all four information segments may be recovered.

In order to implement this technique, the methods already described inreference to flowchart 400 of FIG. 4 and flowchart 500 of FIG. 5 may beused with slight modification. In this case, step 410 of flowchart 400would necessarily include FEC encoding each information and paritysegment to generate two or more FEC blocks per segment. In other words,each segment is divided into two or more FEC blocks prior totransmission over the upstream channel. On the receiving side, step 506would comprise identifying bad FEC blocks within each segment, ratherthan simply identifying bad segments, and step 510 would compriseperforming outer block code decoding to replace bad FEC blocks withineach segment, rather than to replace bad segments.

Where a bad FEC block cannot be recovered through the application of theouter block code, the FEC block, or the segment containing the bad FECblock, can be re-requested from the cable modem. Alternately, arepetition technique, as described below, may be utilized to ensuresuccessful transmission of the FEC block.

b. FEC Block Reconstruction and Repetition Coding

FIGS. 16 and 17 depict flowcharts 1600 and 1700 of methods formitigating burst noise using repetition and FEC block reconstructiontechniques in a transmitting device and a receiving device in accordancewith embodiments of the invention. These flowcharts will be described inreference to an embodiment in which the transmitting device is the cablemodem 108 a described above in reference to FIG. 3, and the receivingdevice is the CMTS 104 described above in reference to FIG. 2. However,the invention is not so limited. For example, in an embodiment, thetransmitting device may be a CMTS and the receiving device may be acable modem. Alternatively, other transmitting and receiving devices maybe used.

The method of flowchart 1600 begins at step 1602, in which the cablemodem 108 a receives a packet of information for transmission over anupstream channel to the CMTS 104. At step 1604, the packet received atstep 1602 is copied, or repeated, to generate a total of N packets,where N is an integer of 1 or more.

At step 1606, the original packet received in step 1602 and the repeatedpackets generated in step 1604 are each FEC encoded into 2 or more FECblocks. In a DOCSIS cable modem system, this step comprises applying aReed-Solomon algorithm to each packet to generate 2 or more FEC blocksper packet. At step 1608, each packet is further adapted fortransmission over the physical (PHY) layer of a cable modem system. Forexample, a preamble may be prepended to the FEC blocks for subsequentburst transmission. As a result of steps 1606 and 1608, each packetbecomes the payload of a separate upstream burst.

At step 1610, the cable modem 108 a separately transmits each burstgenerated in steps 1606 and 1608 over an upstream communication channelof a cable modem system to the CMTS 104.

At the first step 1702 of flowchart 1700, the CMTS 104 receives theupstream bursts transmitted from the cable modem 108 a during step 1610of flowchart 1600. At step 1704, a packet is recovered from eachupstream burst. This step includes FEC decoding of the burst data. Atstep 1706, the bad FEC blocks associated with each packet areidentified.

At step 1708, the recovered packets are passed to the headend MAC 218 ofthe CMTS 104, which attempts to identify at least one complete packetthat has been received successfully from the N packets transmitted fromthe cable modem 108 a. If no complete packets have been receivedsuccessfully, then the headend MAC 218 will attempt to reconstruct theoriginal packet from successfully received FEC blocks from each of the Npackets transmitted from the cable modem 108 a. Provided that the burstnoise or other impairment on the communication channel affecteddifferent FEC blocks of each repeated packet, the MAC 218 will be ableto reconstruct the complete packet from the good FEC blocks of eachpacket. In an embodiment, the headend MAC 218 keeps track of good FECblocks by building a list of successfully received FEC blocks from eachpacket.

c. Alternative FEC Block Reconstruction Embodiments

Alternative techniques for mitigating burst noise using FEC blockreconstruction in accordance with embodiments of the present inventionwill now be described.

In an embodiment of the present invention, the CMTS receives a packetfrom the cable modem and identifies bad FEC blocks within each packet.The CMTS then sends a request to the cable modem to retransmit onlythose FEC blocks that were in error, while storing the good FEC blocks.When the CMTS subsequently receives the retransmitted FEC blocks fromthe cable modem, it reconstructs the complete packet and forwards it forfurther processing. If the retransmitted FEC blocks are still in error,the CMTS may send a second retransmission request, either in the samemanner as before, or perhaps modifying some of the signaling parametersso that the FEC blocks may be retransmitted using a different channel ortime slot relative to the interference.

In accordance with this technique, a protocol is required for the CMTSto request the retransmission of bad FEC blocks. In a cable modemsystem, this protocol may be implemented using the DOCSIS protocol withextensions for data ACKs and/or NACKs, a separate protocol using anout-of-band channel, or an Ethernet protocol whereby the CMTS sends theretransmission request to the cable modem as an Ethernet packet. Anothermethod of retransmitting FEC blocks that were erased is to use theREQ/DAT structure of DOCSIS for retransmitting individual FEC blocks.Several REQ/DAT packets may need to be sent in order to retransmit allFEC blocks that were originally erased.

In another embodiment, if the CMTS receives a packet with one or morebad FEC blocks, it transmits data ACKs and/or NACKs to request that thecable modem retransmit the entire original packet. The CMTS saves theFEC blocks that were successfully received from each attempt andreconstructs the original packet when all FEC blocks have been correctlyreceived.

In a further embodiment, a combination of repetition coding and dataACKs/NACKs are used to implement FEC block reconstruction. In thisembodiment, packets are initially transmitted from the cable modem tothe CMTS using repetition coding as described herein. If the CMTSdetermines that the same FEC block in each of N repeated packets is bad,then the CMTS transmits data ACKSs and/or NACKs to the cable modem torequest that the FEC block be selectively retransmitted and possiblyrepeated as well to increase robustness and minimize latency.Alternately, the initial packet can be sent once, and if it is receivedby the CMTS with bad FEC blocks, those FEC blocks can be retransmittedand repetition coded to improve the chance that the erased FEC blockswill survive transmission the second time.

5. Outer Coding and Packet Header Suppression

The outer coding techniques described above may have an adverse effecton network capacity. Indeed, where the outer coding technique is arepetition code, sending a single packet several times reduces theoverall capacity of the network by the same amount as the repetitionfactor. For example, sending a packet two times will consume twice asmuch capacity as sending it a single time, reducing network capacity to50%. One way to mitigate this reduction in capacity is to utilize packetheader suppression technology in conjunction with such outer codingtechniques to reduce the size of each packet. For example, Broadcom'sPropane™ header suppression technology may be used to offset thebandwidth-consuming effect of repetition coding. Propane™ is an advancedtransport technology for DOCSIS-based cable modem systems that isavailable from Broadcom Corporation of Irvine, Calif.

In the case of small packets, a large percentage of the total packetconsists of protocol headers. These headers are highly structured andcan be coded to a much smaller size in most cases. Application ofPropane™ packet header suppression in a DOCSIS environment results in a50% capacity savings over uncompressed packets. When used in conjunctionwith outer coding for small packets, Propane™ transport technologycompletely mitigates the capacity effects of repetition coding where therepetition factor is N=2. Where the repetition factor is N=3, usingPropane™ transport technology results in network capacity of 66% asopposed to 33% for uncompressed packets.

To use Propane™ transport technology in conjunction with outer coding inaccordance with embodiments of the present invention, a transmittingdevice preferably encodes a packet to be transmitted by the Propane™coding process prior to the application of outer coding (e.g., outerblock coding or repetition coding). At the receiving device, the outercode decoding process preferably reassembles the packet and correctserrors as needed prior to decoding of the packet by the Propane™process, which results in the full-size original packet. Alternately,packet header suppression may be applied by the transmitting deviceafter application of outer coding and by the receiving device prior toouter code decoding.

Relevant packet header suppression techniques for cable modem systemsare also set forth in the following commonly owned, co-pending patentapplications: “Cable Modem System and Method for Supporting ExtendedProtocols,” U.S. patent Ser. No. 09/973,875, filed Oct. 11, 2001 by Bunnet al., “Cable Modem System and Method for Dynamically Mixing ProtocolSpecific Header Suppression Techniques,” U.S. patent Ser. No.09/973,781, filed Oct. 11, 2001 by Bunn et al., “EfficientlyTransmitting RTP Protocol in a Network that Guarantees In Order Deliveryof Packets,” U.S. patent Ser. No. 09/973,872, filed Oct. 11, 2001 byBunn et al., and “Dynamic Delta Encoding for Cable Modem HeaderSuppression,” U.S. patent Ser. No. 09/973,871, filed Oct. 11, 2001 byBunn et al. Each of these applications is incorporated by reference asif set forth herein in their entirety.

6. Outer Coding of Downstream Transmissions in a Cable Modem System

As will be appreciated by persons skilled in the relevant art(s), thetechniques set forth above may be advantageously applied to downstreamtransmissions from a CMTS to a cable modem, even where thesetransmissions are continuous as in a DOCSIS cable modem system. Whilethe DOCSIS downstream implements FEC and interleaving in the PHY layerto combat burst noise, additional burst noise mitigation can be gainedby implementing methods in accordance with the present invention on thedownstream link.

On the DOCSIS downstream, packets for a particular cable modem orservice flow may or may not be transmitted contiguously. Thus, inaccordance with an embodiment of the present invention, outer coding maybe applied to one or more packets with different cable modem or serviceflow destinations, which will serve to increase the robustness of thedownstream link in the presence of severe impairments. In any case, thedownstream PHY will be unaffected by the addition of the higher levelouter coding, except that it may now be required to carry more trafficas bursts carrying parity segments or repeated packets are transmitted.This increased traffic may be offset, however, by the fact that fewerre-transmissions may be required in the downstream due to the resultantmitigation of burst errors.

7. Transmission Scheduling

In accordance with embodiments of the present invention, outer encodedpackets (e.g., block-encoded information and parity segments orrepetition-coded packets) are transmitted in separate bursts, as opposedto bundling of outer encoded packets. The separation of these outerencoded packets in their transmission through the channel is critical inany medium where the success or failure of an entire burst transmission(or portion of transmission in a continuous emission) may occur. Anexample is a burst transmission system where preamble, post-amble, orother synchronization symbols or methods are disbursed around or withinthe information waveform, and wherein an impairment level within thechannel sometimes cause the burst to not be properly or adequatelysynchronized and adjusted at the receiver, so that much of theinformation in the burst is not recoverable at the receiver.

Another example is a transitory impairment, which may impact the mediumso severely that an entire burst is lost or severely corrupted ifcoincident with the rise in the impairment (or in a continuoustransmission, the impairment causes an outage for a significant durationof time). Examples of such impairments include but are not limited tothe many types of fading experienced and described in wirelesscommunications, and in the burst noise environment described herein inreference to cable modem systems.

Thus, in accordance with outer block coding embodiments of the presentinvention, it is critical that the individual bits of each informationsegment are separated into columns and that each row created thereby iscovered with parity checks, creating new parity segments. It is alsocritical that each of these segments be transmitted separately (such asseparate bursts in a bursted scheme, or perhaps separated in time in acontinuous scheme), and received, assembled and processed as individualcolumns at the receiver, and then, after recovering the individualcolumns wherever possible, applying row parity to fill in lost and/orseverely corrupted columns, a row at a time.

C. Example Software Implementation

As discussed elsewhere herein, the above-described techniques or methodsmay be implemented as software routines that are executed, in part, bythe MAC of a cable modem and/or the headend MAC of a CMTS. For example,with reference to the example implementation of the CMTS 104 describedin reference to FIG. 2, the headend MAC 218 performs necessary methodsteps by executing software functions with the assistance of the CPU222. These software functions may be stored in either RAM 220 or ROM218. Furthermore, with reference to the example implementation of thecable modem 108 a described in reference to FIG. 3, the MAC 314 performsnecessary method steps by executing software functions with theassistance of the CPU 320. These software functions may be stored ineither RAM 322 or ROM 324.

However, methods of the present invention need not be limited to theseembodiments. For example, the methods of the present invention may beembodied in software routines which are executed on computer systems,such as a computer system 1800 as shown in FIG. 18. Various embodimentsare described in terms of this exemplary computer system 1800. Afterreading this description, it will be apparent to a person skilled in therelevant art how to implement the invention using other computer systemsand/or computer architectures.

The computer system 1800 includes one or more processors, such asprocessor 1804. The processor 1804 is connected to a communicationinfrastructure 1806. Also connected to the communication infrastructure1806 is a display interface 1802 that facilitates bidirectionalcommunication with a display 1830. The computer system 1800 alsoincludes a main memory 1808, preferably random access memory (RAM), andmay also include a secondary memory 1810. The secondary memory 1810 mayinclude, for example, a hard disk drive 1812 and/or a removable storagedrive 1814, representing a floppy disk drive, a magnetic tape drive, anoptical disk drive, etc. The removable storage drive 1814 reads fromand/or writes to a removable storage unit 1816 in a well-known manner.Removable storage unit 1816, represents a floppy disk, magnetic tape,optical disk, etc., which is read by and written to by removable storagedrive 1814. As will be appreciated, the removable storage unit 1816includes a computer usable storage medium having stored therein computersoftware and/or data.

In alternative embodiments, secondary memory 1810 may include othersimilar means for allowing computer programs or other instructions to beloaded into computer system 1800. Such means may include, for example, aremovable storage unit 1822 and an interface 1820. Examples of such mayinclude a program cartridge and cartridge interface (such as that foundin video game devices), a removable memory chip (such as an EPROM, orPROM) and associated socket, and other removable storage units 1822 andinterfaces 1820 which allow software and data to be transferred from theremovable storage unit 1822 to computer system 1800.

Computer system 1800 may also include a communications interface 1824.Communications interface 1824 allows software and data to be transferredbetween computer system 1800 and external devices. Examples ofcommunications interface 1824 may include a modem, a network interface(such as an Ethernet card), a communications port, a PCMCIA slot andcard, a wireless LAN (local area network) interface, etc. Software anddata transferred via communications interface 1824 are in the form ofsignals 1828 which may be electronic, electromagnetic, optical, or othersignals capable of being received by communications interface 1824.These signals 1828 are provided to communications interface 1824 via acommunications path (i.e., channel) 1826. This channel 1826 carriessignals 1828 and may be implemented using wire or cable, fiber optics, aphone line, a cellular phone link, a wireless link, and othercommunications channels.

In this document, the term “computer program product” refers toremovable storage units 1816, 1822, and signals 1828. These computerprogram products are means for providing software to computer system1800. The invention is directed to such computer program products.

Computer programs (also called computer control logic) are stored inmain memory 1808, and/or secondary memory 1810 and/or in computerprogram products. Computer programs may also be received viacommunications interface 1824. Such computer programs, when executed,enable the computer system 1800 to perform the features of the presentinvention as discussed herein. In particular, the computer programs,when executed, enable the processor 1804 to perform the features of thepresent invention. Accordingly, such computer programs representcontrollers of the computer system 1800.

In an embodiment where the invention is implemented using software, thesoftware may be stored in a computer program product and loaded intocomputer system 1800 using hard drive 182, removable storage drive 1814,interface 1820 or communications interface 1824. The control logic(software), when executed by the processor 1804, causes the processor1804 to perform the functions of the invention as described herein.

In another embodiment, the invention is implemented primarily inhardware using, for example, hardware components such as applicationspecific integrated circuits (ASICs). Implementation of hardware statemachine(s) so as to perform the functions described herein will beapparent to persons skilled in the relevant art(s). In yet anotherembodiment, the invention is implemented using a combination of bothhardware and software.

D. Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention as definedin the appended claims. Accordingly, the breadth and scope of thepresent invention should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

1. A method for transmitting packets of electronic information,comprising: organizing one or more packets into a plurality ofinformation segments; encoding said plurality of information segments inaccordance with a block code to generate at least one parity segment;adapting each of said plurality of information segments and said atleast one parity segment for transmission over a channel of acommunications system, wherein said adapting includes forward errorcorrection (FEC) encoding each of said plurality of information segmentsand said at least one parity segment, thereby dividing each of saidplurality of information segments and said at least one parity segmentinto a corresponding plurality of FEC blocks; and separatelytransmitting each of said plurality of FEC blocks over said channel. 2.The method of claim 1, wherein said adapting further includes adaptingeach of said plurality of information segments and said at least oneparity segment for transmission over a channel of a cable modem system.3. The method of claim 1, wherein said encoding said plurality ofinformation segments in accordance with a block code comprises encodingsaid plurality of information segments in accordance with a Hammingcode.
 4. The method of claim 1, wherein said encoding said plurality ofinformation segments in accordance with a block code comprises encodingsaid plurality of information segments in accordance with an exclusiveor (XOR) code.
 5. The method of claim 1, further comprising addingidentification bits to each of said plurality of information segmentsand to said at least one parity segment to indicate that each of saidplurality of information segments and said at least one parity segmentbelong to a unique group of encoded segments.
 6. The method of claim 1,further comprising adding identification bits to each of said pluralityof information segments and to said at least one parity segment, whereinsaid identification bits indicate an encoding order of said informationsegments and said at least one parity segment.
 7. The method of claim 1,wherein said adapting further includes interleaving each of saidplurality of information segments and said at least one parity segment.8. A method for receiving packets of electronic information, comprising:separately receiving a plurality of information segments and at leastone parity segment over a channel of a communications system, whereineach of said plurality of information segments and said at least oneparity segment comprises a plurality of FEC codewords; forward errorcorrection (FEC) decoding each of said plurality of information segmentsand said at least one parity segment, wherein said FEC decodingcomprises FEC decoding each of a plurality of FEC blocks within each ofsaid plurality of information segments and said at least one paritysegment; identifying one of said plurality of FEC blocks within one ofsaid plurality of information segments as erased, wherein said erasedFEC block includes more errors than can be corrected by said FECdecoding; restoring said erased FEC block by decoding non-erased FECblocks in said plurality of information segments and said at least oneparity segment in accordance with a block code; and organizing saidplurality of information segments into one or more packets.
 9. Themethod of claim 8, wherein said receiving a plurality of informationsegments and at least one parity segment over a channel of acommunications system comprises receiving a plurality of informationsegments and at least one parity segment over a channel of a cable modemsystem.
 10. The method of claim 8, wherein said decoding non-erased FECblocks in said plurality of information segments and said at least oneparity segment in accordance with a block code comprises decodingnon-erased FEC blocks in said plurality of information segments and saidat least one parity segment in accordance with a Hamming code.
 11. Themethod of claim 8, wherein said decoding non-erased FEC blocks in saidplurality of information segments and said at least one parity segmentin accordance with a block code comprises decoding non-erased FEC blocksin said plurality of information segments and said at least one paritysegment in accordance with an exclusive or (XOR) code.
 12. The method ofclaim 8, further comprising reading identification bits in each of saidplurality of information segments and said at least one parity segment,wherein said identification bits indicate that each of said plurality ofinformation segments and said at least one parity segment belong to aunique group of encoded segments.
 13. The method of claim 8, furthercomprising reading identification bits in each of said plurality ofinformation segments and said at least one parity segment, wherein saididentification bits indicate an encoding order of said informationsegments and said at least one parity segment.
 14. The method of claim8, further comprising de-interleaving each of said plurality ofinformation segments and said at least one parity segment.
 15. A methodfor transferring packets of electronic information, comprising:organizing one or more packets into a plurality of information segments;encoding said plurality of information segments in accordance with ablock code to generate at least one parity segment; adapting each ofsaid plurality of information segments and said at least one paritysegment for transmission over a channel of a communications system, saidadapting including forward error correction (FEC) encoding each of saidplurality of information segments and said at least one parity segment,thereby dividing each of said plurality of information segments and saidat least one parity segment into a corresponding plurality of FECblocks; separately transmitting each of said plurality of FEC blocksover said channel; receiving and FEC decoding each of said plurality ofFEC blocks associated with each of said plurality of informationsegments and said at least one parity segment; identifying one of saidplurality of FEC blocks associated with one of said plurality ofinformation segments as erased, wherein said erased FEC block includesmore errors than can be corrected by said FEC decoding; restoring saiderased FEC block by decoding non-erased FEC blocks associated with saidplurality of information segments and said at least one parity segmentin accordance with said block code; and reorganizing said plurality ofinformation segments into said one or more packets.
 16. A device fortransmitting packets of electronic information, comprising: a mediaaccess control configured to receive one or more packets, organize saidone or more packets into a plurality of information segments, and encodesaid plurality of information segments in accordance with a block codeto generate at least one parity segment; physical layer logic coupled tosaid media access control, said physical layer logic configured to adapteach of said plurality of information segments and said at least oneparity segment for transmission over a channel of a communicationssystem, said adapting including forward error correction (FEC) encodingeach of said plurality of information segments and said at least oneparity segment, thereby dividing each of said plurality of informationsegments and said at least one parity segment into a correspondingplurality of FEC blocks; and a transmitter coupled to said physicallayer logic, said transmitter configured to separately transmit each ofsaid plurality of FEC blocks over said channel.
 17. The device of claim16, wherein said physical layer logic is configured to adapt each ofsaid plurality of information segments and said at least one paritysegment for transmission over a channel of a cable modem system.
 18. Thedevice of claim 16, wherein said media access control is configured toencode said plurality of information segments in accordance with aHamming code.
 19. The device of claim 16, wherein said media accesscontrol is configured to encode said plurality of information segmentsin accordance with an exclusive or (XOR) code.
 20. The device of claim16, wherein said media access control is further configured to addidentification bits to each of said plurality of information segmentsand to said at least one parity segment to indicate that each of saidplurality of information segments and said at least one parity segmentbelong to a unique group of encoded segments.
 21. The device of claim16, wherein said media access control is further configured to addidentification bits to each of said plurality of information segmentsand to said at least one parity segment, wherein said identificationbits indicate an encoding order of said information segments and said atleast one parity segment.
 22. The device of claim 16, wherein saidphysical layer logic is further configured to interleave each of saidplurality of information segments and said at least one parity segment.23. A device for receiving packets of electronic information,comprising: a receiver configured to separately receive a plurality ofinformation segments and at least one parity segment over a channel of acommunications system wherein each of said plurality of informationsegments and said at least one parity segment comprises a plurality offorward error correction (FEC) codewords; physical layer logic coupledto said receiver, said physical layer logic configured to FEC decodeeach of said plurality of information segments and said at least oneparity segment, wherein said FEC decoding comprises FEC decoding each ofa plurality of FEC blocks within each of said plurality of informationsegments and said at least one parity segment, and to identify one ofsaid plurality of FEC blocks within one of said plurality of informationsegments as erased, wherein said erased FEC block includes more errorsthan can be corrected by said FEC decoding; and a media access controlcoupled to said physical layer logic, said media access controlconfigured to restore said erased FEC block by decoding non-erased FECblocks in said plurality of information segments and said at least oneparity segment in accordance with a block code and to organize saidplurality of information segments into one or more packets.
 24. Thedevice of claim 23, wherein said receiver is configured to receive saidplurality of information segments and said at least one parity segmentover a channel of a cable modem system.
 25. The device of claim 23,wherein said media access control is configured to decode non-erased FECblocks in said plurality of information segments and said at least oneparity segment in accordance with a Hamming code.
 26. The device ofclaim 23, wherein said media access control is configured to decodenon-erased FEC blocks in said plurality of information segments and saidat least one parity segment in accordance with an exclusive or (XOR)code.
 27. The device of claim 23, wherein said media access control isfurther configured to read identification bits in each of said pluralityof information segments and said at least one parity segment, whereinsaid identification bits indicate that each of said plurality ofinformation segments and said at least one parity segment belong to aunique group of encoded segments.
 28. The device of claim 23, whereinsaid media access control is further configured to read identificationbits in each of said plurality of information segments and said at leastone parity segment, wherein said identification bits indicate anencoding order of said information segments and said at least one paritysegment.
 29. The device of claim 23, wherein said physical layer logicis further configured to de-interleave each of said plurality ofinformation segments and said at least one parity segment.
 30. A systemfor transferring packets of electronic information, comprising: atransmitting device, including: a first media access control configuredto receive one or more packets, organize said one or more packets into aplurality of information segments, and encode said plurality ofinformation segments in accordance with a block code to generate atleast one parity segment, first physical layer logic coupled to saidfirst media access control, said first physical layer logic configuredto adapt each of said plurality of information segments and said atleast one parity segment for transmission over a channel of acommunications system, said adapting including forward error correction(FEC) encoding each of said plurality of information segments and saidat least one parity segment, thereby dividing each of said plurality ofinformation segments and said at least one parity segment into acorresponding plurality of FEC blocks, and a transmitter coupled to saidphysical layer logic, said transmitter configured to separately transmiteach of said plurality of FEC blocks over said channel; and a receivingdevice, including: a receiver configured to receive said plurality ofFEC blocks over said channel, second physical layer logic coupled tosaid receiver, said second physical layer logic configured to FEC decodeeach of said plurality FEC blocks associated with each of said pluralityof information segments and said at least one parity segment, and toidentify one of said plurality of FEC blocks associated with one of saidplurality of information segments as erased, wherein said erased FECblock includes more errors than can be corrected by said FEC decoding,and a second media access control coupled to said second physical layerlogic, said second media access control configured to restore saiderased FEC block by decoding non-erased FEC blocks associated with saidplurality of information segments and said at least one parity segmentin accordance with a block code and to organize said plurality ofinformation segments into said one or more packets.